This invention relates to circuits and more particularly to a system for establishing the input impedance of an amplifier in a stacked configuration.
Prior attempts to match the input impedance of an amplifier to the impedance associated with a load, such as a load resistor, include adding a resistor to the amplifier in either shunt or series configuration. A drawback to this approach is that it adds a 3 dB noise figure in the circuit. A further drawback to this approach is that it is difficult to always set the input impedance of the amplifier and the gain of amplifier independently of each other.
Prior attempts to stack a mixer and an amplifier so that they could share a bias current involved the use of a current source in the circuit. A drawback to this approach is that the voltage used to operate the transistor associated with the current source consumes a valuable portion (e.g., 1.0 to 1.5 volts) of the overall voltage headroom available in the circuit.
In accordance with the present invention, the disadvantages and problems associated with prior circuits have been substantially reduced or eliminated.
In one embodiment, a circuit for establishing the input impedance of an amplifier includes an amplifier, a circuit component, a first feedback resistor, and a second feedback resistor. The amplifier has an input impedance and is coupled to a load having a load impedance. The circuit component is coupled to the load and shares at least a portion of a bias current with the amplifier. The first feedback resistor is coupled to the amplifier and the load, and has a first impedance. The second feedback resistor is coupled to the amplifier and has a second impedance. The input impedance of the amplifier is established based at least in part upon the first impedance and the second impedance.
In another embodiment, a circuit for establishing a bias current includes a mixer, an amplifier, a bias resistor, a comparator, and a drive circuit. The mixer receives a drive voltage and at least a portion of a bias current. The amplifier is coupled to the mixer and receives at least a portion of the bias current. A bias resistor is coupled to the amplifier at a node and a bias voltage exists at the node. The comparator is coupled to the node and compares the bias voltage with a reference voltage. The comparator further generates an output signal based at least in part upon the comparison. The drive circuit generates the drive voltage in response to the output signal such that the bias voltage substantially equals the reference voltage thereby establishing the bias current.